Generally, an electrophotographic process using a photoconductor having photoconductivity is one of information recording techniques utilizing a photoconduction phenomenon of the photoconductor.
According to the process, a surface of the photoconductor is first charged uniformly with electricity by corona discharge in a dark place, and then image exposure is carried out to allow an exposed portion to selectively discharge, thereby to form an electrostatic image on an unexposed portion. Subsequently, colored and charged fine particles (toner) are attached to the latent image by electrostatic attracting force to form a visible image, thereby forming an image.
In such a series of processes, it is demanded that the photoconductor have the following fundamental characteristics:
1) The photoconductor can be uniformly charged up to an appropriate potential in a dark place.
2) The photoconductor has high charge-retaining ability and is reduced in discharge in a dark place.
3) The photoconductor is excellent in photosensitivity and rapidly discharges by irradiation with light.
Furthermore, it is demanded that the photoconductor have the following characteristics in terms of greater stability and durability, for example, charges on the surface of the photoconductor can be removed easily, leaving reduced residual potential; the photoconductor has mechanical strength and excellent flexibility; the photoconductor is not varied in electric characteristics, in particular, in chargeability, photosensitivity and residual potential when used repeatedly; and the photoconductor has tolerance for heat, light, temperature, humidity and ozone degradation.
Since recent electrophotographic photoreceptors that have been put into practical use are each provided with a photosensitive layer formed on a conductive support, carrier injection from the conductive support is likely to occur to cause surface charges to disappear or decrease microscopically, thereby generating an image defect.
To prevent such an image defect, to cover defects on the surface of the conductive support, to improve chargeability, to enhance adhesion of the photosensitive layer and to improve coatability, an undercoat layer (interlayer) is provided between the conductive support and the photosensitive layer.
Conventionally, various resin materials and materials containing inorganic compound particles such as titanium oxide powders have been considered as the undercoat layer.
As the materials to use for forming the undercoat layer with a resin monolayer, resin materials such as polyethylenes, polypropylenes, polystyrenes, acryl resins, vinyl chloride resins, vinyl acetate resins, polyurethane resins, epoxy resins, polyester resins, melamine resins, silicone resins, polyvinyl butyral resins and polyamide resins, copolymer resins including two or more types of these repeat units, and further, casein, gelatin, polyvinyl alcohols and ethyl cellulose and the like are known, among which polyamide resins are particularly preferable (Patent Document 1: Japanese Unexamined Patent Publication No. SHO 48(1973)-47344).
However, with an electrophotographic photoreceptor provided with a monolayer of a resin such as a polyamide as the undercoat layer, the residual potential is greatly accumulated, the sensitivity decreases, and image fogging is generated. Such a tendency is significant particularly under a low-humidity environment.
In order to prevent generation of image defects attributed to the conductive support and improve the residual potential, therefore, there have been proposed an undercoat layer containing surface-untreated titanium oxide powders (Patent Document 2: Japanese Unexamined Patent Publication No. SHO 56(1981)-52757, an undercoat layer containing titanium oxide fine particles coated with alumina or the like to improve the dispersibility of titanium oxide powders (Patent Document 3: Japanese Unexamined Patent Publication No. SHO 59(1984)-93453, an undercoat layer containing metal oxide particles surface-treated with a titanate coupling agent (Patent Document 4: Japanese Unexamined Patent Publication No. HEI 4(1992)-172362) and an undercoat layer containing metal oxide particles surface-treated with a silane compound (Patent Document 5: Japanese Unexamined Patent Publication No. HEI 4(1992)-229872).
However, image properties of the photoconductor according to the methods disclosed in these related art documents are still insufficient, and therefore an electrophotographic photoreceptor having further improved properties has been desired.